1. Field of the Invention
The present invention relates to an image recording apparatus for recording an image on a substantially rectangular sheet-like recording medium, and a sheet material transporting apparatus.
2. Description of the Related Art
FIG. 21 is a sectional view of a conventional sheet material transporting apparatus.
In FIG. 21, a sheet material transporting apparatus conveys the sheet material S in the direction of the arrow A (leftward in FIG. 21) by a pair of transport rollers 1102 to guide it into a U-shaped transport path 1101 having a curved portion 1101a. In this configuration the sheet material S is conveyed above a pair of transport rollers 1102 along the U-shaped transport path 1101.
The transport rollers 1102 are rotatably driven by a drive unit (not shown), so as to subject the surface of the sheet material S to a friction force and advance the sheet material S in the direction of the arrow A.
FIG. 22 is a sectional view of a conventional image recording apparatus including a recording medium transporting apparatus which is similar to the sheet material transporting apparatus of FIG. 21.
In a recording medium transporting apparatus of FIG. 22, a sheet feeding roller 1206 feeds sheet material S stacked on a sheet material stacking unit 1201 into a U-shaped transport path 1202. The sheet material S is guided by the U-shaped transport path 1202 in a U-shape, and then fed to a print unit 1203 disposed above the sheet material stacking unit 1201.
The print unit 1203 is provided with discharging rollers 1207 for holding the sheet material S and feeding it downstream. Downstream of the print unit 1203, a U-shaped transport path 1204 is disposed.
The sheet material S is fed into the U-shaped transport path 1204 by the discharging rollers 1207 after the image-recording, and discharged to a discharging tray 1205 disposed above the print section 1203 while being guided in a curve by the U-shaped transport path 1204.
In this way, the U-shaped transport path is generally used in order to change the transport direction of the sheet material in a sheet material transporting apparatus which conveys the sheet material S to perform a predetermined processing.
Relevant techniques are disclosed in Japanese Patent Application Laid-Open No. 2002-234636 and Japanese Patent Application Laid-Open No. 9-40230.
As shown in FIGS. 23A to 23C, each curved portion of the U-shaped recording medium transport paths 1202, 1204 can have various curvatures. The curvature has a maximum value in FIG. 23A, a minimum value in FIG. 23C, and an intermediate value in FIG. 23B.
The whole apparatus becomes large in size when the curvature is small as shown in FIG. 23C, and the whole apparatus becomes small in size when the curvature is large as shown in FIG. 23A.
On the other hand, if the curvature is as large as shown in FIG. 23A, when the sheet material S is fed into the U-shaped transport paths 1202, 1204, the sheet material S may be strongly inflected so as to buckle, and the sheet material S cannot be conveyed.
Therefore, the curvature of the U-shaped transport path should be preferably as large as possible so long as the sheet material S can be conveyed and pass through the path. In general, the maximum curvature, at which the sheet material S can be smoothly curved along the U-shaped transport path 1202, 1204 when being fed into the U-shaped transport path 1202, 1204, is referred to as “the maximum curvature”.
The factors that determine “the maximum curvature” include the transport speed, the material of the transport path (friction coefficient), the driving force and the location of the transport roller, however, the thickness and the width of the U-shaped sheet material S are the factors that have the most direct impact in determining “the maximum curvature”.
That is, if it is thick or wide, the sheet material S has high rigidity, therefore it becomes difficult to curve the sheet material S. Accordingly, maximum curvature is reduced and, as a result, the size of the whole image recording apparatus becomes larger.
In the above-mentioned conventional sheet material transporting apparatus, the thickness or width of the chosen sheet material defines “the maximum curvature”, which imposes a limit on miniaturization of the apparatus. On the other hand, “the maximum curvature” defines the upper limits of the thickness and width of the conveyable sheet material, if “the maximum curvature” is constant.
The printing apparatus for forming an image on recording paper according to image information can be classified into a heat transfer type, a thermosensitive type, an ink jet type, a wire dot type, and a laser beam type, etc. depending on the image forming method employed by the recording head. FIG. 24 is a diagrammatic vertical sectional view which shows the schematic arrangement of a heat transfer type printing apparatus. In FIG. 24, reference numeral 2101 designates a thermal head as a recording head; reference numeral 2102, a platen roller for guiding and supporting recording paper P through the thermal head 2101; reference numeral 2103, an ink sheet to which a transfer ink is applied; and reference numeral 2104, a pair of sheet feeding rollers disposed downstream in the transport direction of the thermal head 2101 for transporting the recording paper P.
The recording paper P is brought into pressure contact with the thermal head 2101 by the platen roller 2102, and ink of the ink sheet 2103 is transferred to the recording paper P by the heat it produces, while a pair of sheet feeding rollers 2104 is transporting the recording paper P. During this process, the recording paper P is pulled and conveyed in the arrow direction by the sheet feeding rollers 2104, while the recording paper p is pressed into contact with the platen roller 2102 by the thermal head 2101. This allows the recording paper P to be constantly subjected to tension between the thermal head 2101 and the sheet feeding rollers 2104, which leads to accurate conveyance. Printing is performed by the thermal head 2101 on the recording paper P which is thus accurately conveyed. A thermo-transfer type printer using a thermal head is disclosed for example, in Japanese Laid-Open Patent Publication No. 9-93501.
FIG. 25 is a diagrammatic vertical sectional view which shows the schematic arrangement of an ink jet type printing apparatus. In FIG. 25, reference numeral 2201 designates an ink tank for storing ink used for image-forming; reference numeral 2202, an ink jet head as an image forming unit comprising a plurality of discharge ports for selectively jetting ink, supplied from the ink tank 2201, based on image information; reference numeral 2203, a carriage carrying the ink tank 2201 and the ink jet head 2202, and reciprocally moving in the main scanning direction; reference numeral 2204, a guide shaft for guiding and supporting the reciprocally moving carriage 2203; reference numeral 2205, sheet feeding rollers disposed upstream of the ink jet head 2202 in the transport direction, for transporting the recording paper P; and reference numeral 2206, a platen for supporting the recording paper P which is being fed through the ink jet head 2202.
The carriage 2203 is reciprocally driven in the width direction of the recording paper (in a direction vertical to the drawing) by a moving unit (not shown). The recording head 2202 is driven based on the image information in synchronization with the movement of the carriage 2203 so that the ink is jetted from the discharge port train of the recording heads 2202 in accordance with the image in order to print the image on the recording paper P. When the printing corresponding to the length of said plurality of discharge ports (a recording width of one line) is finished, the sheet feeding rollers 2205 conveys the recording paper P by a predetermined pitch, and the recording paper P is stopped at the next line. Then, the carriage 2203 is moved again and the recording head 2202 is driven to print the next line. Thus, the recording of one line and the sheet-feeding by a predetermined pitch are repeated in turn to perform printing on the recording paper P.
In these days, there have been increased requests for printing out image information from digital cameras and digital video cameras, etc., simply in a photographical manner, and printing on the whole recording paper without a margin as in a margin-less photograph. However, in the case where the sheet feeding rollers 2104 are arranged downstream in the transport direction of the recording head 2101 (thermal head) as in the printing apparatus shown in FIG. 24, an unprintable area appears from the leading edge (front edge) of the recording paper P in a range shown by the two arrows X. On the other hand, in the case where the sheet feeding rollers 2105 are arranged upstream of the recording head 2202 in the transport direction, an unprintable area appears from the trailing edge of the recording paper P in a range shown by two arrows Y.
The recording paper P cannot be conveyed unless the recording paper P is nipped by the sheet feeding rollers 2104 and 2205, while the recording paper P cannot be printed unless the recording paper P is being conveyed, as a result the unprintable range appears. Therefore, in the case of FIG. 24, printing is not performed in a range shown by the two arrows X from the leading edge of the recording paper P, whereas in the case of FIG. 25, printing is not performed from the trailing edge of the recording paper P in a range shown by the two arrows Y. As a method of solving the above-mentioned problems, sheet feeding rollers are disposed in both front and back of the image forming unit (recording head portion).
FIG. 26 is a diagrammatic vertical sectional view which shows the schematic arrangement of a printing apparatus in which the thermo-transfer type printing apparatus of FIG. 24 additionally has sheet feeding rollers 2105 upstream of the recording head 2101 in the transport direction, and FIG. 27 is a diagrammatic vertical sectional view which shows the schematic arrangement of a printing apparatus in which an ink jet type printing apparatus additionally has sheet feeding rollers 2207 downstream in the transport direction of the recording head 2202. According to the configuration shown in FIGS. 26 and 27, the recording paper is conveyed through the image forming unit with the recording paper P being nipped constantly by at least one pair of sheet feeding rollers, so that the image can be printed on the whole surface of the recording paper P without making a margin.
However, in a case where the conveyance is carried out by two sets of sheet feeding rollers as shown in FIGS. 26 and 27, the image forming unit is inevitably located between two different sets of sheet feeding rollers. In this configuration in which conveyance is performed by two sets of sheet feeding rollers, the diameter of the roller and the transport speed have to be adjusted in minute detail so as to prevent the image forming unit from losing or excessively increasing the tension of the recording paper. This requires highly accurate machining of components and automatic control, which leads to technical problems such us upsizing of the apparatus or complication of the controlling system.